1. Field of the Invention
The present invention relates to the disposal of industrial waste streams, and more particularly but not by way of limitation, to an improved catalyst-free process for disposing of industrial waste streams containing materials that produce objectionable products when combusted in conventional combustion processes.
2. Brief Statement of the Prior Art
The destruction of industrial waste streams requires the process designer to consider and take into account many factors, and to balance these factors. Many environmental restraints are imposed, and the prior art processes for destroying such industrial waste streams reflect those restraints when dealing with such contaminants as nitrated compounds which produce oxides of nitrogen (NO.sub.x), and with certain halogenated compounds which produce halogen gas.
Many prior art processes utilize a reducing zone into which an industrial waste stream is first injected. An example of such a process is taught in U.S. Pat. No. 3,873,671, issued to Reed et al. and entitled "Process for Disposal of Oxides of Nitrogen".
The Reed process provides for the burning of a hydrocarbon fuel with less than a stoichiometric amount of oxygen. The oxygen can be supplied by air, or by a stream of air containing oxides of nitrogen. The combustion products of the hydrocarbon fuel are then mixed with gases to be treated containing NO.sub.x in a ratio which provides an excess of oxidizable material, under conditions that enable a portion of the combustible products to be oxidized by oxygen made available from the decomposition of the NO.sub.x, thus reducing the NO.sub.x content. This combined combustion mixture of nitrogen and other compounds, i.e., carbon monoxide, hydrocarbons, and other oxidizable materials, is thereafter cooled to a temperature in the range of from about 2000.degree. F. to about 1200.degree. F. with a cooling fluid which is substantially free of oxygen. To prevent venting excess combustibles into the atmosphere, the cooled mixture of nitrogen, combustion products and other oxidizable materials is thereafter mixed in a second zone with sufficient oxygen to convert substantially all of the oxidizable combustion products remaining to carbon dioxide and water while minimizing the reformation of oxides of nitrogen.
In Japanese Patent Application No. Showa 54-50470, published Apr. 20, 1979, a boiler is operated to reduce the NO.sub.x content of a waste combustion gas. In this process a primary fuel is initially burned to produce a waste gas containing NO.sub.x with excess oxygen; a secondary light petroleum fuel is then introduced into the combustion gases to convert the NO.sub.x therein to elemental nitrogen and more excessively reduced forms of nitrogen such as HCN and NH.sub.3 ; and these compounds are then reoxidized back to elemental nitrogen in one or more stages with an oxygen-containing gas.
Other prior art processes have in similar manner taken advantage of the kinetics of combustion control for eliminating or controlling NO.sub.x and the like, such as: U.S. Pat. No. 3,911,083 uses steam and hydrogen injection; U.S. Pat. No. 4,519,993 teaches a process for the safe destruction of an industrial waste stream which contains chemically bound nitrogen compounds without effecting flame propagation; and U.S. Pat. No. 3,867,507 provides a method for removing oxides of nitrogen as air contaminants. An early teaching of flame destruction of nitrous gases by flame combustion is found in British Patent No. 667,342.
Prior art combustion processes usually involve a reducing zone into which the wastes materials are first injected. If the materials are light gases or low boiling liquids, the waste materials can possibly be burned without producing excessive soot. However, if system controls fluctuate, or if heavy gases, vapors, liquids or solids are injected for destruction, soot can and often will be formed. This soot can lead to excessive buildup of coke deposits which can plug off the burner and combustion chambers. If halogens are also present, and if certain temperatures ranges are incurred, dioxanes and/or furans may be formed. This country's federal regulatory code requires for certain toxic wastes that combustion be carried out at temperatures in excess of 2200.degree. F. with at least 3 percent excess oxygen. However, the by-products generated by many such wastes when combusted under these conditions preclude the use of combustion for destroying such wastes.
For NO.sub.x control, a first reducing zone will normally destroy essentially all NO.sub.x by reducing same to elemental nitrogen, providing that the temperature is high enough. As noted above, if free carbon (as particulates) is formed, the burnout of the contaminants then becomes a serious problem. To achieve burnout, the temperature must be greater than about 2000.degree. F. with an excess of oxygen greater than about one volume percent. However, this reoxidation step under these conditions will regenerate NO.sub.x at substantial rates.
Control of the system is very difficult because soot (or smoke) can blind flame detectors and other safety devices which will then shut down the process; furthermore, oxygen analyzers and combustibles analyzers which are used for process control, can become plugged.
Should dioxanes be formed, temperatures of at least 2200.degree. F. and an excess of oxygen of at least three volume percent is recommended by regulatory authorities for adequate destruction of such dioxanes. These conditions, as noted, will regenerate NO.sub.x at unacceptable levels.
What is needed is a process for the safe destruction of waste materials that produce objectionable products when combusted in an atmosphere of excess oxygen. The present inventive process provides this and is well suited for the disposal of hazardous chemicals containing halogenated and nitrated waste materials.